How to Test Exceptions in pytest: Simple Guide with Examples

In pytest, you test exceptions using the raises context manager. Wrap the code that should raise an exception inside with pytest.raises(ExpectedException): to assert that the exception occurs as expected.

Syntax

The basic syntax to test exceptions in pytest uses the raises context manager. You write with pytest.raises(ExpectedException): followed by the code that should raise the exception.

• pytest.raises(): Context manager to catch exceptions.
• ExpectedException: The specific exception type you expect.
• The code inside the with block should raise the exception.

python

import pytest

with pytest.raises(ValueError):
    int('invalid')

Example

This example shows how to test a function that raises a ZeroDivisionError when dividing by zero. The test passes if the exception is raised.

python

import pytest

def divide(a, b):
    return a / b

def test_divide_by_zero():
    with pytest.raises(ZeroDivisionError):
        divide(10, 0)

Output

============================= test session starts ==============================
collected 1 item

test_example.py .                                                        [100%]

============================== 1 passed in 0.03s ===============================

Common Pitfalls

Common mistakes when testing exceptions in pytest include:

• Not using the with block, which causes the test to pass even if no exception is raised.
• Testing for the wrong exception type.
• Putting code outside the with pytest.raises() block that should raise the exception.

python

import pytest

def divide(a, b):
    return a / b

def test_wrong_usage():
    # Wrong: exception raised outside the context manager
    divide(10, 0)  # This will raise and fail the test

    with pytest.raises(ZeroDivisionError):
        pass  # No exception here

# Correct usage:

def test_correct_usage():
    with pytest.raises(ZeroDivisionError):
        divide(10, 0)

Quick Reference

Usage	Description
with pytest.raises(ExceptionType):	Start context to expect an exception
code_that_raises()	Code inside this block should raise the exception
as exc_info	Optional: capture exception info for further checks
assert 'message' in str(exc_info.value)	Check exception message content

Key Takeaways

Use pytest.raises() as a context manager to test exceptions.

Place the code that should raise the exception inside the with block.

Specify the exact exception type you expect to catch.

Avoid putting exception-raising code outside the pytest.raises() block.

You can capture exception details with 'as' to assert on the message.